Gain control circuits are used in electronics wherever there is a need to control the amplitude of one signal with a second signal. For example, fader circuits use a control signal applied to a pair of gain control circuits to increase the gain of one input signal while simultaneously decreasing the gain of a second input signal. Each gain control circuit in a conventional fader circuit typically includes a differential input transconductance stage and a pair of differential amplifiers. The transconductance stage generates current signals in response to an input signal voltage, and these current signals are coupled to the common emitters of the differential amplifiers. Control signals applied to the bases of the component transistors of the differential amplifiers adjust the gain of the corresponding collector currents. By coupling one collector to a bias supply and the remaining collector to a subsequent stage, the signal current generated by the input stage is distributed between the bias supply and the subsequent stage, in accordance with the control signal applied to the transistor bases. Conventional fader circuits combine two such gain control circuits, and apply control signals having opposite polarities to each pair of differential amplifiers to produce the desired signal fading.
Conventional gain control circuits are simple to implement since they use transistors of a single conductivity type. When used as the components of fader circuits, these gain control circuits provide an output that is zero to within the matching of the component devices when the inputs are zero. On the other hand, there are serious limitations to such conventional gain control circuits. For example, the input circuits are class A amplifiers, and consequently must dissipate considerable power to provide wide dynamic input ranges. Increasing the size of input stage resistors to reduce the quiescent current may produce frequency dependent effects due stray transistor capacitances. In addition, the output of a fader circuit based on such conventional gain control circuits is not referenced to ground, and so must be shifted by an additional stage.